Organic electroluminescence was first observed and studied in 1960's (E. Gurnee et al., U.S. Pat. No. 3,172,862 (1965)). In the 1980's, an optimized double-layer structure for an OLED, which employed organic thin films prepared by vapor deposition, was invented (C. W. Tang, U.S. Pat. No. 4,356,429 (1982); C. W. Tang et al., Appl. Phys. Lett. 51, 12: 913 (1987)). A conducting polymer-based OLED or PLED was announced shortly after that (R. Friend et al., WO Patent 90/13148 (1990); R. Friend et al., U.S. Pat. No. 5,247,190 (1993)). Since then, there has been growing interest in the research and development of OLED; the growing interest is mainly motivated by the promise of OLED's use in flat panel display technology.
An OLED is comprised of an organic electroluminescent medium, with thickness of the order of 100 nanometers, sandwiched between two electrodes. The most commonly used device configurations of OLED and PLED nowadays can be generally categorized into three types: single layer, double layer and multilayer. Single layer devices, which are the easiest to manufacture among the three, have only a single electroluminescent layer between the anode and the cathode. In a double layer structure, the two layers are responsible for transporting holes and electrons, respectively. One of the hole-transporting or electron-transporting layers is also the emitting layer. In a multilayer device, an emitting layer is inserted between the hole-transporting and the electron-transporting layers; other layers such as hole-blocking, electron-blocking or layers acting as “steps” to its adjacent layer may also be present.
To improve device performance, novel device configurations and materials for OLED and PLED have continuously been investigated. Therefore, there have been many more new device structures other than the three general configurations named above. However, one of the factors that limits the lowering of the driving voltage and enhancement of efficiency is charge transport (L. S. Hung et al., Mater. Sci. Eng. R. 39: 143 (2002)). Emission from non-targeted species present in the device, especially the host, is another obstacle to overcome before high color purity can be obtained; and full color displays can only be achieved when the colors can be controlled under varying currents. Therefore, the search for non-emitting (in the visible region) materials with high charge mobilities, especially for use in the charge transporting layers and hosts for emitting dopants, has become one of the hottest topics for OLED nowadays.
By far, one of the most widely used hole-transporting material (hereinafter referred to as HTM) in OLED is 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (hereinafter referred to as NPB, S. A. VanSlyke et al. U.S. Pat. No. 5,061,569 (1991)). Although the glass transition temperature (Tg) of NPB at 98° C. is rather low, it is still popular due to the ease of manufacturing. Hence, the search on new HTM has been focused on those materials with high thermal stabilities along with high charge mobilities (L. S. Hung et al., Mater. Sci. Eng. R. 39: 143 (2002)). These approaches to HTM are mainly focused on biphenyl diamine derivatives (Y. Shirota et al., Synth. Met. 111:387 (2000); D. F. O'Brien et al., Adv. Mater. 10, 14: 1108 (1998); K. Yamashita et al., Thin Solid Films, 363:33 (2000)), spiro-linked biphenyl amines (J. Salbeck et al., Synth. Met. 91:209 (1997); U. Bach et al., Adv. Mater. 12: 1060 (2000); U. Mitschke, J. Mater. Chem. 10: 1471 (2000); S. Tokito et al., Thin Solid Films, 363: 290 (2000)), and starburst amorphous materials (Y. Shirota et al., Synth. Met. 111: 387 (2000); I.-Y. Wu et al., Adv. Mater. 12: 668 (2000); I.-Y. Wu et al., Chem. Mater. 13: 2626 (2001); C. Giebeler et al., J. Appl, Phys. 85: 608 (1999)).
The most widely used electron-transporting material (hereinafter referred to as ETM) and/or host material in OLED is 8-hydroxyquinoline aluminum (hereinafter referred to as Alq3, C. W. Tang et al., Appl. Phys. Lett. 51, 12: 913 (1987); B. J. Chen et al., Appl. Phys. Lett. 75, 25: 4010 (1999); R. G. Kepler et al., Appl. Phys. Lett. 66, 26: 3618 (1995)). The popularity comes from its thermal and morphological stability, ease of synthesis, purification and deposition into thin films. It is also molecularly shaped to avoid exciplex formation (L. S. Hung et al., Mater. Sci. Eng. R. 39: 143 (2002)). Alq3 is, however, green fluorescent and requires the use of a hole-blocker (Y. Hamada et al., Jpn. J. Appl. Phys. 40: L753 (2001)) when the emission from another material is desired. Moreover, no blue emission can be obtained from any fluorescent or phosphorescent emitter when Alq3 is used as the host since it is impossible to emit light with energy higher than the band gap of Alq3.
Therefore, the present inventors made extensive investigations to develop a new class of multifunctional and thermally stable compounds with high hole and electron mobilities for use in the different layers of an OLED. They have found that materials based on an inorganic analog of benzene, namely borazine or borazole, can be used. A number of derivatives of borazine are known for other purposes. For example, a low dielectric film based on borazine materials is described in U.S. Pat. No. 6,458,719.